Superconducting magnets used in the systems such as magnetic resonance imaging systems (MRI) and accelerators work at a cryogenic temperature of several tens of K or lower. In general, this cryogenic ambient condition is generated by regenerative cryocoolers such as the Gifford-McMahon (GM) cryocoolers. In the regenerative cryocoolers, appropriate regenerator materials are used in consideration of their temperature dependences of specific heat. Currently, commonly used Gifford-McMahon cryocoolers comprise materials of Cu, Pb and rare earth-based compounds such as HoCu2 or Er3Ni as the regenerator materials working effectively for a temperature range from room temperature to about 100 K, for a temperature range from about 100 K to 10 K, and for a cryogenic temperature range below 10 K, respectively.
In recent years, Pb has been considered to be a material which has a high burden on the environment, and a system which does not use Pb is required. Before now, materials such as Bi, Sn and their compounds were proposed as substitutes for the Pb regenerator material.
However, there has been a problem that the materials such as Bi, Sn and the compounds containing them as main components have insufficient specific heat at low temperatures, and thus the cooling properties of the cryocoolers using regenerators filled with such materials as the regenerator materials are inferior to those of cryocoolers using Pb.
In the refrigeration cycle of regenerative cryocoolers such as Gifford-McMahon cryocoolers, pulse tube cryocoolers and Stirling coolers, high-pressure working gas reciprocally flows through the void in regenerator materials filled in a regenerator. Since the heat generated during the compression-expansion cycle of the gas in the cryocoolers is accumulated in the regenerator material to achieve cooling from room temperature to cryogenic temperature, rapid heat transfer from gas to the regenerator material is desired. Thus, efficient heat transfer between the gas and regenerator material, and heat diffusion in the regenerator material are important, so that the regenerator material is required to have high thermal conductivity. Further, in the case of Gifford-McMahon cryocoolers and Stirling cryocoolers, regenerators themselves filled with regenerator materials also move reciprocally. Therefore, regenerator materials preferably have high mechanical strength not to be broken while running the refrigeration cycle.